The present invention relates to a method of interpolating a video signal and apparatus for carrying out the method. The present invention is particularly concerned with a method of interpolating video signals generated and transmitted by sampling such that positions of the picture elements whose signals are sampled are interleaved in two scanning lines adjacent to each other in the same field, and with an apparatus for carrying out the method.
The development of a high definition television system is now under way which has scanning lines about twice as many as those in the current television broadcasting system and also ensures high horizontal-resolution proportionate to the number of the scanning lines. However, in high definition television, which requires transmission of a video signal with a frequency band wide enough for its high horizontal-resolution, it is difficult to carry out such transmission because of the limit of the capacity available of the current transmission means.
Therefore, a signal-band compression method such as MUSE (Multiple Sub-Nyquist Sampling Encoding) is in general use, in which sub-Nyquist sampling with interleaving in frames and fields is used. In MUSE shown in FIG. 3, a sampling point (picture element whose signal is sampled) on a scanning line is shifted along the horizontal direction by half the sampling interval from another sampling point on an adjacent scanning line in the same field. (The sampling interval is for example a space between white circles in FIG. 3.) Interlacing is performed in successive two fields, and the signals representing sampling points are interpolated in successive four fields. With video signals related to each other in the above fashion it is possible to reproduce a still picture showing high resolution thanks to the interpolation of the signals representing sampling points but only to reproduce a picture in motion showing much lower resolution because of no use of signals other than those in the present field. To achieve high resolution for pictures in motion also, such conventional methods have been proposed in Japanese Patent Application Laid -Open No. 64-29183.
In these conventional methods, pictures in motion are generally reproduced by obtaining the signal at a sampling point, which it is impossible to obtain in the present field, from the signals at sampling points adjacent to it (hereinafter referred to also as adjacent points) in the present field. For instance, the signal at point a shown in FIG. 3, which cannot be obtained in the first field, is interpolated by using the adjacent points b and c on the same scanning line or the adjacent points d and e on the scanning lines adjacent to each other in the same field.
In the above conventional methods, in the case of an object image which FIG. 4A shows has an edge portion in parallel with scanning lines, a desired signal can be obtained by interpolating the signal at point x by using the average of the levels of the preceding and subsequent signals on the same scanning line, but cannot be obtained by interpolation by using the average of the levels of the signals for the preceding and subsequent scanning lines, with the result that the reproduced picture has a failure signal as shown in FIG. 4B. By contrast, in the case of another object image which FIG. 4C shows has an edge portion perpendicular to scanning lines, a desired signal can be achieved by interpolating the signal at point y by using the average of the levels of the signals for the preceding and subsequent scanning lines, but cannot be achieved by using the average of the levels of signals on the same scanning line, with the result that the reproduced picture has a failure signal as shown in FIG. 4D.
In terms of frequency domain, the above will be described in the following way:
Let the horizontal frequency spectrum for the video signal obtained at the period of horizontal scanning of the n-th scanning line in FIG. 4A be a spectrum shown in FIG. 5A. In FIGS. 5A to 5P, fs stands for a sampling frequency in the horizontal direction for each scanning line. The hatched part of the spectrum denotes the original horizontal components of the signal for an object, and the other part of the spectrum denotes the side band components of the sampling signal which have occurred because of sampling. In general, since a frequency band of the video signal which has been subjected to the sampling is restricted under the sampling frequency fs through an appropriate low-pass filter and the sampled video signal components whose frequencies are lower than the sampling frequency are utilized for reproducing the image of the object, the lower side band components of the sampling signal may cause a failure signal in the reproduced image of the object.
Thus, the frequency spectrum for the video signal obtained at the period of horizontal scanning of the (n +1)-th scanning line in FIG. 4A is a spectrum shown FIG. 5B with the side band components at fs inverted in the phase by comparison to FIG. 5A, since sampling points on adjacent scanning lines are interleaved. Note that the spectra shown in FIGS. 5A and 5B apply when there is a high correlation between the frequency components of the video signals obtained at the n-th and (n+1)-th horizontal scanning period as they are obtained from the n-th and (n+1)-th scanning lines which are adjacent to each other in FIG. 4A. The relation between these two spectra is true of arbitrary two continuous scanning lines shown in FIG. 4C. FIG. 4A applies when the greater part of the original components of the signal for an object is a direct current component, and FIG. 4C when the frequency components of the signal for an object have a wide frequency band. On the other hand, the frequency spectrum for the video signal obtained from the (n-1)-th scanning line in FIG. 4A, as shown in FIG. 5C, represents a contraction in level of the video signal obtained from the (n+1)-th scanning line, whose frequency spectrum is shown in FIG. 5B.
Now, when the signal obtained at the n-th horizontal scanning period is interpolated by using the average of the levels of the preceding signal and the subsequent signal on the same scanning line, the interpolation signal has a frequency spectrum shown in FIG. 5D. When this signal is combined with the signal shown in FIG. 5A, the signal resulting from the interpolation has the frequency spectrum shown in FIG. 5E. In FIG. 5E, among the side band components having occurred on both sides of fs, some are removed which are caused by the low-frequency components of the signal for the object, but the others, which are high-frequency components, are left. Therefore, a desirable interpolation can be made in the case of the image of the object shown in FIG. 4A in which the greater part of the frequency components is a direct current component, but a failure signal due to side band components occurs in the case of the image of the object shown in FIG. 4C the frequency components have a wide frequency band.
When in the case of the object shown in FIG. 4A the signal obtained at the period of horizontal scanning of the n-th scanning line is interpolated by using the average of the levels of those signals for the preceding and subsequent scanning lines which have been obtained at the periods of horizontal scanning of the (n-1)-th and (n+1)-th scanning lines, the interpolation signal has a frequency spectrum shown in FIG. 5F. This interpolation signal, combined with the signal shown in FIG. 5A, changes into the interpolated signal whose spectrum is shown in FIG. 5G. As in FIG. 5G, the interpolated signal shows the original frequency components a little smaller than before interpolation and also shows a small level of the side band components on both sides of fs left all over the frequency band. Therefore, in the case of FIG. 4A, a failure signal occurs, and a desired signal cannot be obtained.
When a signal obtained at the n-th horizontal scanning period is interpolated by using the average of the levels of the preceding and subsequent signals on the same scanning line and those of the signals for the preceding and subsequent scanning lines at the (n-1)-th and (n+1)-th horizontal scanning period, the frequency spectrum is the average of the frequency spectra shown in FIGS. 5E and 5G. The failure signal occurring there is also a signal having an averaged level, resulting from the mixture of the failure signals occurring in the two cases.
As described above, the conventional means have the disadvantage that a failure signal occurs in the edge portion of the object image in the case of a picture in motion because of the processing only in the field.